Add macports-friendly library and include search paths to Makefile

This is a rather dumb way of doing it an assumes the compiler being
used will behave like gcc and search successive paths in order.

Short of cmake or autoconf I have been unable to find an idiomatic
solution to platform-specific paths.
Merge pull request #15 from agentultra/master

Add macports-friendly library and include search paths to Makefile